This invention relates to electrical motive power control systems, and, more particularly, to positional synchro systems having power amplifier current limiting.
Devices which rely generally on magnetic coupling for measuring the difference between a desired shaft angle and a controlled shaft angle, and convert the difference into an electrical actuating signal are known variously as synchros, differential transformers, E transformers, linear transformers, and pickoffs, depending on their construction.
Synchro systems are used extensively in avionics. Excessive power dissipation in synchro systems is a persistent problem. For example, if a transmitting synchro and an identical receiving synchro or repeater are independently set to the same angular position, their respective rotors excited and the stators connected together, neither unit experiences an increase in rotor excitation current until one of the rotors is moved from the original position. When either unit is so rotated, the power in each rotor increases by the same amount. If, however, one or more additional identical repeater synchros are connected in parallel with the first two, the power in each synchro rotor will depend upon the angular departure of that rotor from the average position of all the synchros. This is because the restoring torque on any one synchro tends to move the one synchro toward a position corresponding to a stator field vector. The stator field vector is the resultant of all the field vectors of the synchros connected to the system. The power necessary to produce the restoring torque must come from the rotors of the synchros and each rotor supplies a portion of the power corresponding with the degree of angular displacement from the resultant field vector position. Consequently, in a system having more than two identical synchros, one of the synchros moved from a common at-rest position must supply most of the power to move the other synchros, and, if due to abnormal friction the other synchros fail to reach the desired positions, the transmitting synchro will continue to supply more power to the system than any of the other synchros. On the other hand, if all except one of the repeater synchros move easily and the one repeater lags considerably due to friction, the other synchros will take up a position approximately half way between the positions of the transmitter and the defective repeater, in which case the transmitter and the defective repeater will each supply half the total power in the system. In prior art systems employing multiple repeaters, the transmitting synchro is larger and has more heat dissipating area and lower internal resistance than other synchros in the system. Even with a transmitting synchro considerably larger than any of the repeaters, the heating of the transmitter may be the limiting factor in the load capability of the system, rather than reduced accuracy limiting the load on the system.
When a synchro transmitter is coupled to one or more repeater synchros, the stator line currents will normally be extremely small because the repeater synchros are pulled to the resultant position, reducing these line currents to small values. Thus, the additional power dissipated by the transmitting synchro is only slightly greater than the power that would be dissipated by the same transmitting synchro if the stator leads were open-circuited. Thus, it would be theoretically possible to add an infinite number of direct driven repeaters without increasing the load on the transmitting synchro if all of the repeaters were initially set to the proper common position before power was first turned on. It is this last requirement that may actually limit the number of repeater synchros which can be driven by a transmitter, viz.: the number of synchros which can be turned 90.degree. to the position of the transmitter and pulled into synchronism by the transmitter when the power is first applied. Further, a synchro system is limited by a requirement that any one repeater synchro in the system should be incapable of causing a failure in any part of the synchro system as a result of a locked rotor.
A typical synchro power amplifier, e.g., for driving three to five synchro loads, dissipates considerable power even when the synchro loads are matched to the synchro driver. Power can easily double for mismatched conditions. Protection of the drive circuit against large synchro drive requirements, i.e., those caused by a sticky synchro, severe phase or amplitude mismatch, etc., was provided in the prior art by a current limiting resistance in series with the load. The resistance comprised a tungsten filament lamp having a rating such that it became luminous when the load current increased to a limiting value, thereby additionally providing a visual indication of the overcurrent condition. See, for example, U.S. Pat. Nos. 2,734,160 and 4,106,013.
The design approach of the prior art was to build high current synchro power amplifiers on metal boards utilizing heat sinks for maximum heat transfer; stated otherwise, mismatch conditions were simply tolerated. The current-limiting resistor and light bulb, while effective for limiting current, contributed to the problem of excessive power dissipation.
It is therefore an object of the instant invention to provide an improved synchro system having current control.
It is another object of the present invention to provide an improved synchro system having power amplifier current limiting.
It is a more particular object of the present invention to provide improved current control of power amplifiers in a synchro system through a control element which emulates a synchro transmitter, measures instantaneous current in the synchro receiver stators and alters input signals to the servo amplifiers to control the stator currents.